If you spent some time on astronomy forums, you may have seen people asking for some help, usually posting an image that is supposed to describe the problem better. The online community is always ready to help, but some comments are more valuable than others.

“Tilt”

When the answer has less than 3 words, the probability of the comment to be insubstantial is high. Aberration, vignetting, tilt, collimation, flats? What does that mean and are those topics related? We will try to demystify all this.  

Coma aberration

Let’s start with a common and simple misunderstanding. A coma is a common aberration and it is the result of refraction differences on the different zones of a lens or a mirror, as shown in the image below.

Source: https://www.handprint.com/ASTRO/ae4.html

The previous definition sounds obscure? Then just keep in mind that an aberration is a property of an optical system and can’t be corrected by a better adjustment of our telescope. However, tilt and collimation issues can make it worse.

Vignetting

Vignetting is only the gradual reduction of the image brightness from the center to the edges. Nice way to say: bright in the middle, less bright on the corners. So why do photons prefer to hit the center of the sensor, rather than the corners? Due to geometrical reasons as shown in the picture below. Note that the phenomenon remains the same, no matter if the light rays are refracted by a lens or reflected by a mirror.

Source: https://www.rp-photonics.com/vignetting.html

To make it simple, oblique light rays may or may not hit the sensor, and the more oblique they are, the less chance they have to hit our photosites. Bad luck.

“Ok but if I understand well, the peak of light in my flat field must be centered, right? So what do I get this this type of master flat below?”

In a perfect world, every piece of your scope must be aligned. But in reality, the sensor can be off-axis for several reasons. Maybe the sensor isn’t perfectly centered in the camera, or the focuser is too weak, or the telescope suffers from a serious flexure from its own weight, or maybe the telescope isn’t collimated properly. Or a combination of them!

Collimation

In this perfect world, primary mirror, secondary mirror, coma corrector and sensor are perfectly aligned. Consequently, a light ray hitting the center of our primary mirror perpendicularly to it will perfectly stay in the middle of the secondary mirror, in the middle of the coma corrector and finally hit the very middle of our sensor.

But what if the secondary mirror of our Newton telescope isn’t exactly angled at 45°, but a little bit less, or a little bit more?

Well then, your perfectly-centered light ray won’t hit the sensor in its center. Is it a problem? It depends! Remember our vignetting issue described before. The more distance from the optical center, the less light. If your scope isn’t perfectly “aligned” (Please say collimated, you’ll look more professional), your precious photons will prefer to hit a black surface instead of the brave sensor.

Flat field

“So, will my flat field compensate all that?”

Of course, aberration issues such as coma errors can’t be. Vignetting (with or without a perfect collimation) can be compensated by adequate flats, but with one important condition: the off-axis error and its orientation must remain the same.

Imagine now that the telescope tube is too weak and has an significant flexture, now you surely understand what the problem could be.

Let’s imagine that we made our flats with the telescope pointing to North, and we capture the image with the telescope pointing to South. The off-axis error will change to the opposite side, and the flat field will either overcompensate on one side, or under-compensate on the other side. This is probably what happened in the following example:

Tilt

“Tilt”.

Thanks AstroKing, but this forum’s answer has no value at all. What is tilted? The primary mirror? The secondary mirror? The coma corrector? The sensor itself?

Now let’s analyze different cases.

• Mirror tilt: No matter which mirror is tilted, it is due do a collimation issue. A minor mirror tilt can possibly be compensated by a good master flat, but shall we? If the tilt is too high, the image quality in the corners will always be worse.

• Focuser tilt: Any tilt between the focuser and the sensor can have a negative influence on the image quality. If you thought that your focuser is perfectly aligned with the 3 adapters and two shims you added for the adjustment of the backfocus, you are probably wrong! A few micrometers difference on its surface, and you are uncentered.

• Sensor tilt: purely due to the camera design quality. Some high-quality cameras are delivered with a tilt adjustment, but the hardest part is to know which screw to screw…

You said rotator?

About the importance of stiffness and flat fields

Now we understand why we can reasonably deal with acceptable and reproducible errors, that can be compensated by a good master flat. And when I say good, I mean good!

An acceptable error can be a minor collimation error, or a sensor slightly mounted off-centered, for example.

The error can be reproducible if, and only if the static parts (telescope tube, adapters, mirror holders, etc) are stiff enough and the moving parts (focuser, rotator, filter wheel) are high-quality items.

• ·         Is a carbon tube worth? Yes.
• ·         Is an extra-large dovetail worth? Yes.
• ·         Is a high load focuser worth? Yes.
• ·         Are 2” accessories better than 1,25” ones? Yes.
• ·         Are threaded adapters better than sliding connections? Again, yes. 
• ·         Are those items more expensive? I’m afraid yes.

Sorry if those FAQs made you take a bank loan, but that’s the hard truth.  

What can I do to prevent these errors?

Buy the right thing.

Especially if you are a beginner, do not worry about it. We all bought a cheap item online one day and regretted it few weeks later. Premium equipment bought at your favorite astronomy store is rarely a bad option.

Ask the world.

I frequently underestimated this step. Remember you are probably not the only one who uses an F/4 Newtonian telescope with a ZWO camera, or an 80ED with a digital camera… Is my vignetting normal? Is my coma aberration ok? Maybe, maybe not.

Sanity check of your collimation.

Before verifying the collimation with dedicated instruments, just image a bright start out of focus and ensure that the flare is centered. If one of your filters have a strong reflection (typically OIII or SII), use it to your advantage and check the centricity of the reflection flare.

Identify the origin of the tilt

It might be a difficult task but turning each adapter by 90° can help to identify where the tilt come from. Recent software can measure the FWHM in the whole picture and evaluate the tilt and its orientation. You turned the manual rotator and your tilt changed? Maybe it is time to open your toolbox and file the 1mm spacer.

Take your time.

I know. It’s like telling someone who’s angry to calm down. Optical problems can have several root causes and are therefore hard to identify and solve. Play your favorite music, make a good coffee, take a paper and pencil. After all, it’s a cloudy night with full moon today…